The present invention relates to a thermal transfer recording apparatus and to a method for thermally transferring ink on an ink sheet onto recording paper and performing multicolor display.
Conventionally, a variety of apparatuses and methods for doing multicolor printing with a thermal transfer recording apparatus have been realized.
FIG. 6 is an explanatory diagram showing a printing mechanism used in a thermal transfer recording apparatus, and hereinafter operation of this mechanism will be explained in connection with fusion type thermal transfer.
An ink sheet 21 and a sheet of recording paper 22 are carried so as to be sandwiched between a thermal head 23 and a platen 24. The thermal head 23 is pressed against the platen 24 so that the ink sheet 21 and heat generating resistive members of the thermal head 23 contact each other satisfactorily. The recording paper 22 is carried so as to be sandwiched between a capstan roller 25 and a pinch roller 26. The capstan roller 25 is normally/reversely rotated by a driving motor which is not shown in FIG. 6 while the capstan roller 25 is pressed against the pinch roller 26, thereby feeding the recording paper 22 forward/backward. In the above state, while the capstan roller 25 is normally rotated to feed the recording paper 22, the thermal head 23 is energized to fuse ink on the ink sheet 21 and transfer the ink onto the recording paper 22. Then, the ink sheet 21 is peeled off the recording paper 22. Generally, yellow is printed first, and then the capstan roller 25 and the platen 24 are reversely rotated to carry the recording paper 22 back to a first position. Next, magenta and cyan are printed on the yellow in this sequence in the same way as described above so as to pile the three colors, thereby realizing multicolor printing.
A fusion-type binary recording apparatus displays each dot in accordance with respective binary signals (ON, OFF) for three colors including cyan, magenta and yellow, so that a density pattern method, a systematic dither method and so on are applied to obtain an image with density level. In the density pattern method, dots are arranged in a matrix such as a two by two or four by four matrix and pseudogradation is obtained in accordance with an area ratio to display one pixel. In the systematic dither method, one pixel is displayed with one dot while each dot is multiplied by a dither matrix to obtain psedogradation. Therefore, print recording time is so short that print recording can be obtained at a low running cost.
However, it is necessary to arrange a matrix in a larger size in order to increase the number of colors to be displayed. For example, a matrix size of 16 by 16 is required to record each color in 256 density levels. When one pixel is displayed with this matrix of 16 by 16, if a resolution of the thermal head is 300 dpi, a size of the matrix becomes approximately 1.4 square mm. Therefore, this case has a defect that gradation of a small image in a halftone cannot be displayed satisfactorily.
In a sublimation type recording apparatus, a density level can be changed by the pixel. Therefore, if the ink sheet is composed of three colors: cyan, magenta and yellow, when each color is recorded in n density levels, n cubed kinds of colors can be displayed for each pixel. If each color is recorded in, for instance, 256 density levels, approximately 16,700,000 colors can be displayed. Therefore, this type of apparatus can realize clean print recording and is frequently applied to print recording of images. However, it has defects of long print recording time, high running cost and so on.
The sublimation type recording apparatus can change a density level one pixel by one pixel. On the other hand, the fusion type multi-density level recording apparatus can change an area onto which ink is transferred by the pixel. When the ink sheet includes three colors of cyan, magenta and yellow, if each color is recorded in n density levels, n cubed kinds of colors can be displayed for each pixel. If each color is recorded in, for instance, 256 density levels, approximately 16,700,000 colors can be displayed. Therefore, the fusion type apparatus can realize clear print recording and is frequently applied to print recording of images as well as the sublimation type one. However, it has a defect in that print recording takes longer than the fusion type binary recording apparatus.
FIG. 7 is a graph showing density level characteristics which are observed when a line cycle for energizing a thermal head is 10 ms and 3.5 ms respectively. A line cycle of 3.5 ms is a general time taken to printing-record a sheet in 60 seconds with the fusion type binary record apparatus. In this graph, the horizontal axis represents a color density signal of image data to be printed, and the vertical axis represents a percentage dot area of ink transferred. As apparent from the graph, when the line cycle is 10 ms, a satisfactory density level characteristic can be obtained. On the other hand, when the line cycle is 3.5 ms, while image data is between 11h and 88h, a satisfactory density level characteristic can be obtained. However, when the image data is beyond 88h, dot-join rapidly occurs, when the image data becomes 99h or AAh, a percentage dot area rapidly increases; and when beyond AAh, dots become almost crushed. Therefore, a successive density level characteristic cannot be obtained while the percentage dot area is between 60% and 90%.
The cause of a rapid occurrence of the dot join phenomenon is that the thermal head and the ink sheet accumulate heat because of the fast line cycle and even the ink originally not intended to be transferred is likely to be transferred onto the recording paper, which cannot be improved by precise control of an energizing pulse width of the thermal head. In order to obtain a desirable successive density level by preventing the dot join, there are regularly provided dots which are not printed, for instance, two dots or one dot among every four dots in a 2.times.2 matrix, and the dots to be printed are driven with a large energy, thereby thermally diffusing printing ink around the dots and filling up an unprinted dot region with the diffused ink.
FIGS. 8A and 8B are diagrams showing an example in which one dot among every four dots is not printed and the other three dots are used for density-level recording. An energy amount of the three dots is controlled with the identical color density signal so that a percentage area becomes the same as that of an ordinary density level recording, thereby obtaining a satisfactory density level characteristic.
When a single color is printed by a conventionally proposed method for preventing dot join, a satisfactory density level characteristic can be obtained. However, when cyan, magenta and yellow are overlaid one on top of another for multicolor printing by this method, white dots, that is, unprinted dots on recording paper which exist regularly for all the colors to be displayed are visually recognized as noise of high frequency and deteriorate image quality remarkably.
When even a single color is printed, regular white dots exist on the recording paper as well as for multicolors, however, the amount of noise which is recognizable is so small that printing quality does not deteriorate. As the number of colors to be displayed for multicolor printing increases, the amount of white dots recognized as noise seems to increase, the cause of which is presently unclear.